Reinforced abrasive wheel and method of making same



May 15, 1956 J. R. ERICKSON 2,745,224

REINFORCED ABRASIVE WHEEL AND METHOD OF MAKING SAME Filed May 12, 1955 2Sheets-Sheet 1 3 INVENTOR.

EH v R. Enmxsa- A TTOENE Y y 1955 K J. R. ERICKSON 2,745,224

REINFORCED ABRASIVE WHEEL AND METHOD OF MAKING SAME Filed May 12, 1955 2Sheets-Sheet 2 JOHN R. ER/C'KSUN y United States Patent Company,Worcester, Massa, a corporation of Massachuset'ts Application May 1955,start No. 507,895

2 Claims. (Cl. 51-495 The invention relates to reinforced abrasivewheels particularly adapted for grinding, by the'use of a portable handtool on' which the abrasive wheel is mounted for rotation thereby, sucha wheel usually having a depressed center so that the nut holding it tothe spindle of the tool will not project beyond the face of the wheel.An example of this type of'wheel is shownand described in my U. S.Letters Patent No. 2,656,654 of October 27, 1 953. This application is acontinuation-impart of my copending application Serial No. 459,771 filedOctober 1, 1954, now abandoned.

One object of the invention isJto provide an abrasive wheel of this typehaving, all'of' the following. characteristics and features:

Another object of the invention is to provide an abrasive wheel having.all of the above six characteristics and features with the safety web offeature No.. 5 capable of holding. a segment at a speed of 50% above themaximum operating speed set by the manufacturer whensuch segment is.deliberately broken back to an angle of 90 to the rear face of thewheel'.

Another object of the invention is to provide abrasive Wheels of thenature indicated particularly suitable for use in smoothening the weldsin automobile bodies or on other fabricated metal and insmoothening'protuberances, irregularities or rough spots on any kind ofmetal; Another object of the inventionisto provide an abrasive wheel totake the place of coated abrasive discs which have only a single layerof abrasive, thus eliminating frequent replacement of 'theabrasive'element. Another object of the invention is to provide anabrasive'wheel of the above type which will quickly take. on a bevel ormitre on the periphery since grinding with a portable hand tool of thetype herein illustrated-is often done with the wheel held at anacuteangle to the face of the: work. Another object of the invention is.to provide a wheel capable of grinding as in the preceding object andalso capable of use as a cutting-off wheel. Another object of theinvention is. to provide a wheel with one or more of the characteristicsindicated which has a good abrasive action on the rear side sincefrequently, for one reason or another, the; workmen wish touse the rearor back side. of thewheelafter it has been beveled through use becausethereby they can grind hollow spots, grooves or notches as the wheel isheld. with the front side at an obtuse angle to the workpiece. Anotherobject is to provide a wheel suitable for removing rust and scale fromsteel plate; Another object is tov provide a general 'all purposegrinding'wheelespecially adapted for use on a portable hand tool of thekind herein illustrated and described.

Other objects will be in part obvious or in part pointed outhereinafter- In the accompanying drawings illustrating a preferredprocedure for manufacturing a Wheel according to the invention and thewheel so produced together with a representation of the power tool uponwhich it may he used.

Figure 1 is an exploded view illustrating in section the severalcomponents of the wheel in the raw state,

Figure 2- is a cross sectional view of a mold partly filled, that iscontaining one of the components of Figure 1,

Figure 3 is a cross sectional view of the mold of Fig ure 2 completelyfilled and closed, that is the components having been pressed to uniteto form the wheel,

Figure 4 is a plan view of the completed wheel,

Figure 5 is a cross sectional view of the wheel after it has acquired abevel or mitre together with the mount-' ing disc therefor,

Figure 6 is a perspective view of a power hand tool such as used forsmoothening welds, etc., having the wheel of the invention securedthereon.

Referring first to Figure l, I provide a quantity of dry granular mix 10having organic bond. This corn prises abrasive grains each granule ofwhich is coatedwith the organic bond. As a practical matter the abrasivegrains will usually be either aluminum-oxide grains or silicon carbidegrains. However other abrasives such as garnet or even diamonds can beused. Aluminum oxide abrasive is available in several different gradesincluding the so-called regular grade which is a brown abrasive about95% A1203, the white porous variety which is about 98% pure or betterand containsa small percentage of soda, and recently developed aluminumoxideabrasive produced in' discrete crystals and being usually betterthan. 99% A1203. Silicon carbide abrasive is also available in severaldiiferent grades such as the black grades and: the green, the latterbeing the purest grade; All this is known to the art and any variety ofthese or other abrasives can be used.

While so far as the broad aspects of the invention are concerned manydifferent organic bonds might be used I prefer to use availablecommercial grades of phenol formaldehyde resin otherwise known asresiuoid; Unconverted phenol formaldehyde resin is available to themanufacturer in" two forms, the solid phase material and the liquidphase material. For the production of a: dry granular mix in which eachabrasive granule is coated:

. with an organic bond, the abrasive is placed in a mixing pan and thenwet with a liquid and then, while continuing the mixing, bond inpowdered form is added and eventually the abrasive coated with liquidpicks up the powdered bond until each granule has an envelope of bondand the entire mix appears to be fairly dry even though it. does containliquid phase material. Thus the abrasive grains can be wet with liquidphenol formaldehyde resin and then solid phase powdered phenolformaldehyde resin can be added to complete the formation of the drygranular: mix. On the other hand the abrasive grains can be wet withfurfural and then powdered resin added. The powdered phenol formaldehyderesin available and which: is hereinafter identified contains a minorpercentage of cresol formaldehyde.

In. describing an illustrative embodiment of the manufacture of a wheelaccording to the invention, I shall describe the manufacture of a Wheelnine inches in diameter, one-quarter inch thick and with a seven-eighthsi nch hole, the-wheel having an off-set hub.

For the manufacture of this wheel a dry granular mix can be made fromthe following ingredients:

EXAMPLE I Dry granular mix ingredients Pounds No. 24 grit size aluminumoxide abrasive, regular In making the mix the abrasive is placed in amixing pan and mixing is started whereupon the furfural is added. Whenall of the abrasive is wet a mixture of the cryolite, lime, carbon blackand the powdered phenol formaldehyde is slowly added and mixing iscontinued until the powder is well distributed. Then the anthracene oilis added. This has the function of laying the dust. The cryoliteimproves the grinding action of the wheel, makes it stronger and alsoimproves the consistency of the mix. With this small amount of furfuralin the mix about 80% of the bond is loose in the mix. The carbon blackgives a uniform color, to wit, black. The lime absorbs any water whichmay be liberated.

Referring now to Figures 2 and 3, I provide a steel mold consisting of acylindrical mold band 11, a bottom plate 12, a top plate 13, and anarbor 14. For the manufacture of a wheel with an offset center, thebottom plate 12 and the top plate 13 can be shaped as shown. Both ofthese plates have knurled surfaces in the form of concentric ratchetteeth from the periphery to one and a half inches from the center. Forthe manufacture of a 9 inch diameter wheel the mold band 11 has aninside diameter of 9% inches since the cured wheel is trued to size.Further description of the mold would seem to be superfluous since thedrawings are clear and molds of this general character are well known toeverybody concerned with the manufacture of grinding wheels.

Figure 2 shows the mold with the mold band 11, the bottom plate 12 andthe arbor 14, the top plate 13 being absent. The mold in this conditionis filled with the various ingredients to be pressed as follows:

I first place in the mold on top of the bottom plate 12 a piece of wovenglass cloth 15 six inches in diameter with a 2 /2 inch central hole.This glass cloth is a very open plain weave material. It has a threadcount of 10 picks to the inch and 10 sley (warp ends to the inch) and abreaking strength in both directions that is along the filling and alongthe warp of approximately 400 pounds per inch of Width. In order toavoid fraying of the glass yarn (threads) at the edges of the glasscloth discs it is coated with starch. This glass cloth, besides thecoating of starch, has a superimposed coating of the same BR2417 brandof powdered reactive phenol formaldehyde resin as is mentioned above.The resin is simply brushed onto the glass cloth while the latter is atroom temperature. Each nine inch disc with the seven-eighth inch centralhole (hereinafter mentioned) will pick up two grams of this resin. Dueto the fact that the filling and the warp of the cloth have appreciablediameter the openings therein are about one-sixteenth of an inch square.Thus even though the glass cloth is on both sides of the wheel themolding operation forces the abrasive grain, being of No. 24 grit size,right through the openings to the sides of the wheel. Figure 2 shows theopen mold without the top plate 13 having been charged only with thedisc of glass cloth.

Next I add 80 grams of the mix of Examplel and 4, follow this with anine inch in diameter, seven-eighths inch central hole layer of the sameglass cloth 15 also coated with starch and BR2417 resin to the sameamount per square inch. I then add an annular sheet 16 nine inches indiameter with a seven-eighths inch hole of porous technical paper No. 69of the Industrial Paper Company. This paper is made of regeneratedcellulose and wood pulp with the fibres in random orientation. Itsporosity cannot be measured on a densimeter test as defined by theTechnical Association of the Pulp and Paper Industry, as no readingswere obtained even with the lightest (5 ounce) cylinder used. This paperis approximately .003 inch thick. It weighs about .75 ounce per squareyard.

On top of the sheet of paper just described I place an annular sheet 18of nylon woven cloth. This is nine inches in diameter with aseven-eighths inch central hole. This nylon cloth has a plain weave witha sley of and with 34 picks to the inch. I The ravelled tensile strengthof the warp is 460 pounds,-of the filling 430 pounds. The tongue teartest'of the warp is 39.2 pounds and of the filling 36 pounds. Thepercentage elongation of the Warp is 35.3% and of the filling 33.5%. Thecloth as a whole has a thickness of about .018" and the weight is 8.5ounces per square yard.

The annular sheet or disc of nylon woven cloth is then followed by 185more grams of the mix 10 of Example I. This of course is spread level aswas the 580 grams below it and then another annular piece of the sameglass cloth 15 six inches in diameter witha two and a half inch diametercentral hole is added. The top plate is now inserted on top of thematerial above mentioned and the assembly is taken to a hot press. Themold and contents are then pressed for about 15 minutes at a temperatureof 160 C. at a pressure of from 500 to 1,000 pounds per square inch.After this the mold is cooled in the press and then removed from thepress whereupon it is stripped from the cured wheel. The pressure is notexactly critical although a fairly substantial pressure should be used,for example 500 pounds or more. If the mix is very flowable, a lowdegree of pressure should be used, otherwise resin will ooze out betweenthe top plate and the mold band. In general I have found that thepressure should be not higher than 1500 pounds per square inch and notless than 300 pounds per square inch. In practice the movement of thetop mold plate 13 is measured and when an average figure for the bestwheels is found mold stops are set to close the mold to the stopsregardless of slight variations in pressure involved in so doing.Molding to stops is now per se well known in the art.

The completed wheel 20 is shown in Figures 4, 5 and 6. It is a disc withan offset hub 21 and a central hole 22, a front face 23 and a rear face24. After grinding the welds on automobile bodies, its periphery usuallyacquires a taper 25 but prior to grinding it has a square edge periphery26. I The faces 23 and 24 are desirably knurled and this may be achievedas follows: the mold plates 12 and 13, being made of'a soft steel suchas cold rolled steel, are mounted in alathe and then a knurling tool isfed against the face of the plate to make a ring of knurling and thenthe knurling tool is moved inwardly or outwardly and the operation isrepeated until the flat portion 27 outward of the offset forming portion28 has been knurled all over its surface. Figure 4 shows the knurling 30thus illustrating the pattern on the plates. I prefer ratchet teethknurling. This knurling of the faces gives the wheel a better appearancebut additionally it improves its action when used as a cut-01f wheel.

The glass cloth discs 15 gives strength to the wheel so that it is foundit does not break until a speed of 30,000 surface feet per minute isreached. It is well known that grinding wheels of a given compositionbreak at a given speed measured in surface feet per minute regardless ofvarying diameter provided the central hole is proportionately the samesize in the different wheels, so therefore a breakingspccd of so manysurface'feet per minute has a real meahi'ng eventhoughthe diameter isnot specified and "the variationdu'e to different ratios of the diameterof the central hole is usually a small" variation.

Referringnow to Figure 6, a typical power driven hand tool 35501- usingthe wheel 20 is' shown. There are many varieties or hand tools that'canuse the wheel of this invention', but to give a typical example of theuse of the wheel I have provided a perspective view of a particular handtool 35- which comprises a metal casing 36 containing a motor having ashaft which drives bevelled gearing in a casing 34 attached to thecasing 36 and the bevelled gearing drives a spindle 37 which has athreaded end 38 receiving a nut' 39 which holds the wheel 20 against ahub 40 integrally connected to a back plate 41. Power is supplied to thetool 35 by electric cables 42 and the operator holds the hand tool 35 bymeans of handles 43 and 44'. By means of this or'any other kind of handtool the Wheel 20 can be used for any of the purposes hereinbeforegiven.

The thickness of the abrasive section 10 on the grinding side of thewheel, that is to say the abrasive mixture which was first put into themold and is the lower one in Figure 1, will vary depending upon thethickness of the wheel. In the case of a one-quarter inch thick wheelthethickness. of the abrasive section on the grinding side is threetimes that of the abrasive section on the back side which is in theupper position in Figure 1. In the case of a one-eighth inch thick wheelthe thickness of the abrasive sectionson the two sides are equal.Inother words, the nylon sheet 18 is. in the same position relative tothe back of the wheel in the several varieties.

Nylon is the condensationproduct of hexamethylenediamine and. adipic'acid. It is a polyamide. It can be called polyhexamethylenediadipi'mide.It will be noted that both components of nylon contain six carbon atoms.The chains link together and" as nylon filament is made it is drawn tostretch the material several times its original length which. lines upthe chains to give the filament great strength. It not only has greattensile stength, of the order of a hundred thousand pounds to the squareinch, but'italso has high elongation up to 40% or more. However, otheryarn can be substituted for nylon such as Dacron and Orlonprovided ithas tensile strength of atleast fifty thousand pounds per square inchand an elongation of at least 15%. So-called regular nylon is listed inthe. tables as having a tensile strength in pounds per square inch of 68to 85- thousand, Orlon from 62 to .71 thousand and regular Dacron from81 to 88 thousand. The corresponding elongations as given in the tablesare respectively 26%32% for regular nylon, l%-'17% for Orlon and19%'23-% for regular Dacron. However, the nylon that'I have had has anelongation of 33%.

It has been found that no matter how strong these grinding wheels aremade they can become cracked. Wheels made according to thepresentinyention can be placed between two bricks and then stood upon bya 200 pound: man without. breaking. They can be dropped fromazconsiderable. height onto a concrete. pavement without breaking.Nevertheless other types of wheels have become cracked at times. Thisshould be a rare occurrence with the present wheels since the threelayers of glass cloth greatly strengthen the wheels. Thetensile strengthof glass is around 210,000 pounds to the square inch. I have found thatthe discs of this open weave glass cloth 15- do not detrimentally affectthe cutting performance of the wheel. This is because of their locationin the wheel and also because glass is itself an abrasive. It is readilydisintegrated by the grinding action. Also it will be noted that the topsheet or disc of glass cloth is not reached until the wheel has beenworn down practically to. discard size where the wheel is being used atan angle of 30 degrees to the work. Discard size for a nine inchdiameter wheel which has been described is about four inches indiameter.

: A critical feature ofxthis wheel isin the bonding of the abrasivephenolic resin mixture to the nylon cloth 18. If the paper 16 wereomitted a cracked segment of wheel could hardly be bent back at allbefore the nylon cloth would break clean. This is because the resinbonds very well to nylon. If this bond were spoiled completely, forexample by using a sheet of cellophane in place of the paper, the wheelwould give short life due to the lack of adhesion and spalling away ofthe abrasive mix at the cellophane edge. With the use of the paperherein described, I obtain just the right strength of bond between thenylon cloth 18 and the lower or abrasive side mix 10.

Wheels made according to the invention have been deliberately broken andthe broken segments bent back to 90 degrees from the bodies of thewheels. Such wheels were then speed tested to breakage with thefollowing results:

In the above table the wheel size is given in inches, first for thediameter, then for the thickness and then for the central hole. S. F. P.M. M. O. S. means surface feet per minute maximum operating speed as setby the manufacturer. The manufacturer has every inducement to set themaximum operating speed as high as. possible consistent with safety toshow the merits of his wheel. The figures given are rather highoperating speeds. Ca1- culation will show that the broken wheel segmentwhich was bent back 90 degrees did not tear loose from the wheel untilthe wheel speed was better than 50% greater than the maximum operating.speed set by the manufacturer.

These wheels made in accordance with the present invention have grindingcharacteristics superior to wheels made in: accordance with my priorPatent No. 2,65 6,654. Wheels made accordingto that patent werecertainly good wheels; hundreds of thousands of them have now been sold.They gave extreme satisfaction and were superior to all other wheels onthe market in my belief. Nevertheless this invention represents afurther improvement. Standard wheels according to Patent No. 2,656,654were tested against a standard wheel according to the present invention.The wheel of the present. invention, as made according to the foregoingdescription, is harder acting than the standard wheel according to theforegoing patent. Experience indicated that the customers would like alittle harder wheel which would not remove material quite so fast butwould last a good deal longer. The material removal is of course afunction of the pressure used and the operators appeared to want aharder acting wheel which would nevertheless remove the welds quicklyenough for their purposes. A wheel that cuts too quickly can cut rightinto the metal of the automobile body thus causing a great deal ofexpense to the automobile manufacturer. The grinding data of Tables 11and III are taken right out of my copending application referred to butthe tests were made on an outer. portion of the invention wheel which instructure was identical with the invention wheel of the inventionhereof. Therefore, the same figures are used.

WW means wheel wear and MR means material removed. Dia. means diameterand grns. means grams. The quality number is simply the square of thematerial removed divided by the wheel wear as will be found bycalculation from the other figures given. This particular quality numberhas no relationship to the quality numbers in other units which will befound in the literature. It is not arbitrary, however, because bysquaring the material removed an allowance is made which has been foundto be valid for the operators pay and the manufacturers overhead. Thesuperiority of the wheel of the present invention is shown both by theefficiency and by the quality number and is also shown by the smallerwheel wear. Both wheels were 7 x A x wheels.

Another test was made comparing the hard wheel of the present inventionmade as above described against a standard cotton carded web filledabrasive wheel which is in regular production and is preferred by manycustomers for various purposes. This standard cotton carded web abrasivefilled wheel has been on the market for a number of years and manythousands thereof have been sold. These wheels are also 7 X M4 2; and acomparison in the same units is shown in the following table:

Table III lg?! MB in Efiiciency, Quality males Gms. M R/WW NumberStandard Cotton Carded Web Filled Wheel 375 290 775 224, 266 Wheel ofPresent Invention .250 460 1,840 846, 400

Table IV [Diameters of components of wheels in inches] Ov ra OutsideGlass Cloth, Hole. Inside Glass Cloth, OveralL. Inside Glass Cloth, Ho1ePorous Paper, Overa11 Porous Paper, H Woven Nylon Cloth, Overall. WovenNylon Cloth, Hole It will be seen from the drawings, Figure 1, from thedescription hereinbefore given and especially from Table IV that theporous paper and the glass cloth are interposed between the nylon clothand the resinoid bonded abrasive. This gave me a chance to make tests ofthe adhesion between the nylon cloth and the resinoid bonded abrasivemix by cutting discs out of a wheel made in accordance with thisinvention. These discs were actually one inch in diameter and they werefastened to parallel metal plates and then the tensile strength wastested on an Olsen testing machine. Araldite cement was used as thebonding material for joining the discs to the metal plates. A specialwheel was made just for the purpose of testing in which there was onlythe glass cloth between the nylon cloth and the resinoid bondedabrasive. One inch diameter discs were cut from this wheel and cementedto parallel metal plates and then tested on the Olsen testing machine.All results were translated to pounds per square inch and are given inthe following table, many tests having been made. The wheels areidentified as wheels 1 and 2 of which wheel 1 was a wheel according tothe invention, and wheel 2 wasthe special wheel with only the glasscloth between the nylon cloth and the resinoid bonded abrasive. Fourdiscs were cut from each wheel and all of the discs were cut from nearthe periphery but the results should be in the same range if they werecut elsewhere outside of the hub. At the end of the table the figuresare totaled for convenience and under that the average strength isgiven. Below that will be found the ranges of strength.

Table V [Tensile strength of adhesion of nylon cloth to resinoid bondedabrasive in pounds per square inch.]

With Porous With only Paper and the Glass Glass Cloth Cloth betweenbetween Wheel Nylon Cloth Wheel Nylon Cloth and and Resinoid ResinoidBonded Bonded Abrasive Abrasive Total 3, 286 Total 3, 985

Average 821. 5 Average- 996. 25

Table VI Tensile Strength in Pounds per Square Inch of ZoneCorresponding to Inner Zone Hereoi Wheel Total Average In certain of thetests reported in the copending application the tensile strength of theinner zones of some wheels thereof was found to average 499 poundspersquare inch and I am content with a low limit of my inner zone of 500pounds per square inch as some wheels made according to my invention ofmy aferosaid copending application delaminated in use and I believe thatwas because their inner zones had a tensile strength below 500 poundsper square inch.

The broken segments reported in Table I were broken across the outerpart of the inner zone about half way from the periphery tothe centralhole on nine inch diameter wheels. It will be seen from Table V thatwheel 2 was bonded with a strength which in some places was less thanthe strength of bonding of wheel 1 in some places. Consequentlyitfollows that useful wheels can be made with the porous paperterminating inside of the full diameter of the wheel, for example at sixinches in Table IV. While these wheels will not be quite so resistant toflying apart if broken by a blow, they will be superior to prior Wheelsand might be used. Satisfactory wheels according to the invention can bemade if in an inner zone defined as three inches less in diameter thanthe ing tensile strength in the inner zones at the lower limit (500) andat the upper limit (1090) and having tensile strength in the outer zonesat the lower limit (500) and at the upper limit (1140) have been made asin part pointed out in my copending application referred to. The paperor some substitute, either in two sheets or one, need not extend rightto the central hole although I prefer it that way, but it Will sutficeif there is paper between the glass cloth and the woven cloth in atleast 90% of the'area of the inner zone. There could be a disc of paperin the inner zone and none in the outer zone, or one in each but ofdifferent characteristics, but the preferable arrangement is as shownand described.

In the illustrative embodiment the woven cloth disc is approximately.060 inch from the back side of the wheel. It can be defined as nearerto one side than to the other.

It will thus be seen that there has been provided by this invention areinforced abrasive wheel and method of making same in accordance withwhich the various objects hereinabove set forth together with manythorougly practical advantages are successfully achieved. As manypossible embodiments can be made of the above invention and as manychanges might be made in the embodiment above set forth, it is to beunderstood that all matter here- 10 inbefore set forth or shown in theaccompanying drawings is to be interpreted as illustrative and limitedonly by the scope of the claims.

I claim:

1. An abrasive wheel comprising a disc with a hole centrally locatedtherein, said disc being made of abrasive grains bonded with a maturedphenol formaldehyde resin, and said disc containing three discs of Wovenglass cloth, one on each side and one intermediate, and a disc of wovencloth between the intermediate glass cloth and one of the glass clothdiscs on the side of the wheel, said woven cloth being made of yarnhaving a tensile strength of at least 50,000 pounds per square inch andan elongation of at least 15%, and paper between the intermediate glasscloth disc and the woven cloth disc, the latter being bonded to theabrasive grains bonded with phenol formaldehyde through the glass clothdisc at an inner zone defined as three inches less in diameter than thediameter of the wheel with a tensile strength of from 500 to 1090 poundsper square inch and bonded at an outer zone defined as the outer halfinch of the wheel on the radius with a tensile strength of from 500 to1140 pounds per square inch, there being paper between the intermediateglass cloth and the woven cloth in at least of the area of the innerzone, the woven cloth being nearer to one side of the Wheel than to theother.

2. An abrasive wheel according to claim 1 in which the inner zone andthe outer zone are bonded together with the same tensile strength.

No references cited.

1. AN ABRASIVE WHEEL COMPRISING A DISC WITH A HOLE CENTRALLY LOCATEDTHEREIN, SAID DISC BEING MADE OF ABRASIVE GRAINS BONDED WITH A MATUREDPHENOL FORMALDEHYDE RESIN, AND SAID DISC CONTAINING THREE DISCS OF WOVENGLASS CLOTH, ONE ON EACH SIDE AND ONE INTERMEDIATE, AND A DISC OF WOVENCLOTH BETWEEN THE INTERMEDIATE GLASS CLOTH AND ONE OF THE GLASS CLOTHDISCS ON THE SIDE OF THE WHEEL, SAID WOVEN CLOTH BEING MADE OF YARNHAVING A TENSILE STRENGTH OF AT LEAST 50,000 POUNDS PER SQUARE INCH ANDAN ELONGATION OF AT LEAST 15%, AND PAPER BETWEEN THE INTERMEDIATE GLASSCLOTH DISC AND THE WOVEN CLOTH DISC, THE LATTER BEING BONDED TO THEABRASIVE GRAINS BONDED WITH PHENOL FORMALDEHYDE THROUGH THE GLASS CLOTHDISC AT AN INNER ZONE DEFINED AS THREE INCHES LESS IN DIAMETER THAN THEDIAMETER OF THE WHEEL WITH A TENSILE STRENGTH OF FROM 500 TO 1090 POUNDSPER SQUARE INCH AND BONDED AT AN OUTER ZONE DEFINED AS THE OUTER HALFINCH OF THE WHEEL ON THE RADIUS WITH A TENSILE STRENGTH OF FROM 500 TO1140 POUNDS PER SQUARE INCH, THERE BEING PAPER BETWEEN THE INTERMEDIATEGLASS CLOTH AND THE WOVEN CLOTH IN AT LEAST 90% OF THE AREA OF THE INNERZONE, THE WOVEN CLOTH BEING NEARER TO ONE SIDE OF THE WHEEL THAN TO THEOTHER.